Element immersed in coolant of nuclear reactor

ABSTRACT

The zirconium cladding of a coolant-displacement rod of a nuclear reactor is precollapsed in the zirconium oxide stack of pellets which supports the cladding. Current is conducted through the cladding in an atmosphere at reduced pressure containing residual oxygen, to heat the cladding to a temperature at which its yield strength is reduced. Then, while the rod remains at this temperature, it is subjected to isostatic pressure which collapses the cladding uniformly. The formation, by reason of exposure to neutron flux, of a long unsupported gap in the cladding which might be collapsed under the pressure of the coolant is precluded. In addition, the rod retains its symmetry. The outer surface of the cladding is oxidized, facilitating the movement of the rod into its thimbles of the core and improving the resistances of the cladding to reaction with the coolant.

This application is a continuation of application Ser. No. 07/163,140filed Feb. 24, 1988 now abandoned, which is a continuation ofapplication Ser. No. 06/802,563 filed Nov. 27, 1985 now abandoned, whichis a division of application Ser. No. 06/570,487 filed Jan. 13, 1984,now U.S. Pat. No. 4,606,109 granted Aug. 14, 1986.

Application Ser. No. 570,551, filed Jan. 13, 1985, now U.S. Pat. No.4,725,401 to Robert K. Gjertsen et al. for "Element Immersed in Coolantof Nuclear Reactor", now U.S. Pat. No. 4,725,401 assigned toWestinghouse Electric Corporation, is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to nuclear reactors and it has particularrelationship to the elements of a nuclear reactor which are immersed inits coolant. Among such elements are fuel elements and control elementsof fuel assemblies. In addition, there are coolant-displacement rods.Coolant-displacement rods are immersed in the coolant of a nuclearreactor when the nuclear-reactor plant is started up and at thebeginning of each refueling cycle (typically 18 months). They reduce themoderation of the neutron flux, which would be produced by the coolant,by displacing the coolant. After the reactor has been in operation forsome time (typically 10 months of each cycle), thesecoolant-displacement rods are withdrawn from the coolant.

The nuclear reactor elements each includes a stack of pellets enclosedin cladding. A fuel element includes a stack of pellets of fissionableor fertile materials, such as uranium, plutonium or thorium, or certainof their compounds, enclosed in cladding. The cladding may be azirconium alloy or stainless steel. A control element includes pelletsof a neutron absorbing material, such as natural boron or boron 10 ortheir compounds, enclosed in cladding. A coolant-displacement rod, whichis herein also categorized under the heading "element", includes a stackof zirconium-oxide pellets enclosed in cladding of a zirconium alloy.The above-identified Gjertsen et al. patent discloses acoolant-displacement rod in which a portion of the outer surfaces ofcertain of the pellets distributed along the stack are depressed.

In the interest of dealing in concrete concepts, this invention isdescribed in detail herein as applied predominantly tocoolant-displacement rods in which the outer surfaces of all pellets arealike, typically circularly cylindrical as disclosed in Gjertsen et al.patent. This invention is uniquely applicable to such rods. To theextent that this invention is applicable to nuclear-reactor elements ofother types, such application is within the scope of equivalents of thisinvention. Coolant-displacement rods are used predominantly in nuclearreactors of the pressurized-water type. This invention is not confinedto pressurized-water reactors and to the extent that it or itsprinciples are embodied in reactors of other types, such embodiment iswithin the scope of equivalents thereof.

It has been found that when a coolant-displacement rod is exposed tohigh energy neutron-flux, the cladding of zirconium alloy grows orcreeps axially while the zirconium oxide remains dimensionally stable.This growth of the cladding is permanent and progressive. The claddingdoes not retract axially, as happens for thermal expansion when the rodis removed from the flux environment. The permanent growth increases asthe exposure to the flux progresses. The life of a reactor is about 40years. Over this time interval, an unsupported gap of as much as 7inches may be developed in the pellet stack usually at the upper end ofthe rod. At the temperature, pressure and flux of the coolant in thereactor, cladding at the unsupported gap would collapse. Distortion ofcollapsed cladding would cause jamming of a rod in its guide tube orthimble, not only restricting the axial movement of the rod with thecollapsed cladding, but also restricting the movement of the wholeassociated coolant-displacement-rod assembly. Typically there are 24rods in an assembly. Restriction of the movement of these rods wouldmaterially deteriorate the operation of the reactor.

Because of the demands imposed during the manufacture of thecoolant-displacement rods, there is a small clearance, typically 0.006to 0.008 inch, between the inner diameter of the cladding and the outerdiameter of the pellets. In the operation of a nuclear reactor, theserods are exposed, not only to neutron flux, but also to coolant at ahigh temperature, typically 600° F., and at a high pressure, typically2250 pounds per square inch. Under these conditions, the cladding mayengage the pellets non-uniformly, thus degrading the operation of thereactor.

It is an object of this invention to prevent the formation, in acoolant-displacement rod of a nuclear reactor, of an excessively longunsupported gap, which might collapse in the environment of the coolantof the reactor, as a result of the growth or creep of the cladding whenexposed to neutron flux. It is also an object of this invention topreclude coolant-displacement rods from becoming non-uniform intransverse cross-section when exposed to the coolant of a nuclearreactor during operation. A more general object of this invention is toprevent a cladded element which is immersed in the coolant of a nuclearreactor from forming unsupported gaps under the cladding and fromsuffering radial deformation as a result of the exposure of the elementto neutron flux.

SUMMARY OF THE INVENTION

In accordance with this invention the cladding of a coolant-displacementrod, or of other element immersed in the coolant, is pre-collapsed onthe pellets, i.e., brought into engagement with the pellets, as a finalstep in the making of the element. The pellets are packed tightly in thestack. When an element so made is exposed to neutron flux, the cladding,in engagement with the pellets, resists axial expansion and anunsupported gap is not formed.

In the practice of this invention the cladding of the element is heatedto an elevated temperature at which its yield strength is substantiallyreduced. Then, while the cladding is at the elevated temperature,pressure is applied to the element to collapse the cladding on the stackof pellets. The pressure is applied isostatically by a gas whichenvelopes the element. The cladding is collapsed uniformly so that theelement maintains its radial symmetry. Since the processing of theelement in accordance with this invention is the last step in itsmanufacture, each completed element may be inspected for radial symmetryand non-symmetric elements rejected. Reliability of the operation of thenumerous elements in a reactor may be assured. The cladding is, in thepractice of this invention, heated in an atmosphere containing oxygenand its external surface is oxidized. The movement of the element in itsreceptacles or thimbles in the core of the reactor is facilitated andreaction of the cladding with the coolant is minimized.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of this invention, both as to itsorganization and as to its method of operation, together with additionalobjects and advantages thereof, reference is made to the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIG. 1 is a view in side elevation, partly in longitudinal section,showing a coolant-displacement rod which is processed in the practice ofthis invention;

FIG. 2 is a view in transverse section taken along line II--II of FIG.1;

FIG. 3 is a view in side elevation, partly in longitudinal section, of apellet of the coolant-displacement rod shown in FIG. 1;

FIG. 4 is a view in isometric with part of the wall broken away ofapparatus for processing the rod shown in FIGS. 1 through 3 inaccordance with this invention; and

FIG. 5 is an exploded view in isometric showing the manner in which thecladding is connected to be heated by conduction of electrical current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 through 3 show a coolant-displacement rod 11. The rod 11includes a stack of pellets 13 interposed between a tapered end plug 17and a mid plug 19. The pellets 13 are of hollow circularly cylindricalshape. The mid plug 19 terminates in a chamfer 20 at its inner end. Therod 11 is immersed in the coolant with the end plug 17 downwardly. Theexpression "outer end" means the end towards the end plug 17, "innerend" means the end away from this end plug. A spacer 21 is interposedbetween innermost pellet 13 of the stack and the mid plug 19. The endplug 17 is solid. The mid plug 19 is in the form of a hollow circularcylinder terminating at its inner end in a central hole 27 of smalldiameter.

The pellets are enclosed in cladding 29 which extends from a stem 31 atthe inner end of the end plug 17 to a position along the mid plug 19.The end plug 17, the mid plug 19. The spacer 21 and the cladding 29 arecomposed of an alloy whose composition is predominantly zirconium. Atypical such alloy is ZIRCALOY-4 whose typical composition is presentedin the Gjertsen et al. patent.

At its outer end, the cladding 29 is welded pressure tight to theshoulder of the end plug 17 formed by the stem 31. At its inner end, thecladding is welded pressure tight to the surface of the mid plug 19.When the welding is completed, the air is exhausted through hole 27 fromthe space enclosed by the cladding and the space is back-filled with aninert gas such as helium at about atmospheric pressure. The hole 27 issealed pressure tight by a weld. Reaction of components of air such asoxygen and water vapor internally with the ZIRCALOY alloy is thusprecluded.

At the end opposite the end 17, the rod 11 includes a plug 33 having anextension 34 terminating in a threaded tip 35. The tip 35 serves toconnect the rod 11 to the rod drive (not shown). The plug 33 has a stem37 defining a shoulder. A tube 39 extends between this shoulder, towhich it is welded, and the mid plug 19 and abuts the weld 41 betweenthe cladding 29 and the mid plug 19. The tube 39 is secured to the midplug 19 by circular swaged joints 43. The plug 33 and the tube 39 arecomposed of stainless steel. The tube 39 is vented.

The rod 11 is immersed in the coolant with the tapered end 17downwardly. The taper 103 facilitates the entry of the rod into thethimble which receives the rod. The whole rod including the tube 39 isbathed by the coolant. Typically, the pressure of the coolant is about2250 pounds per square inch. The temperature of the coolant while thereactor is in operation is about 600° F. Since the tube 39 is vented,the coolant penetrates into the tube and there is no differential inpressure across the tube wall. There is substantial pressure against thecladding 29. The thermal expansion of the pellets 13, the cladding 29and the tube 39 is reversible. The axial expansion of the cladding whichresults from the flux is permanent. Over the life of a reactor in whicha rod 11 is installed, the expansion would produce an unsupported gapunder the cladding 29 which typically may be as long as 7 inches. Underthe pressure of the coolant and at its temperature the cladding over thegap would collapse. In this state, the cladding might prevent thecoolant-displacement assembly from moving upwardly or downwardly. Inaddition, the creep of the cladding would produce non-uniformity in thetransverse cross-sectional dimension of the rod 11 along its length. Thepurpose of this invention is to prevent such maloperation. This purposeis achieved by processing the rod 11 with the apparatus 51 shown inFIGS. 4 and 5.

This apparatus 51 includes an elongated pressure vessel 53 formed ofhollow circularly cylindrical sections 54 joined by pressure-tightflanges 55. Pressure-tight flanges sold under the name GRAY-LOC pressureflanges or the equivalent may be used. The sections 54 are essentiallypipes composed of mild steel joined by the flanges 55. Near its lowerend, the vessel 51 is provided with an exhaust outlet 57, an inlet 59for supplying gas to pressurize the vessel and an outlet 61 fordepressurizing the vessel.

The cladding 29 is heated by conducting electrical current through it.The current is supplied through upper-terminal assembly 63 andlower-terminal assembly 65 (FIG. 5). The upper-terminal assembly 63includes a feed-through 67 (FIG. 5) for producing a pressure-tight jointat the upper end of the vessel 51. A CONAX feed through or theequivalent may be used. The feed-through 67 includes a hexagonal head 69connected to a conical threaded member 71 with which a nut 73 meshes.The member 71 engages a thread (not shown) in the upper end of thevessel 51 (FIG. 4) and when tightened by the nut 73 produces apressure-tight joint. At its lower end the feed-through 67 carries aneyelet 75 which is secured to a clevis 77 formed of abuttingsemicircular plates 79 from each of which an eyelet 81 extends. Theeyelet 75 is bolted to the clevis 77. The clevis 77 is connected to anexpansion joint 83 including flexible conductors 85 connected betweeneach plate 79 and a plate 87. The upper-terminal assembly also includesa plurality of strips 89 and a clamping unit 91 (FIG. 5). The clampingunit 91 includes clamping jaws 93 having semicircular seats 95,dimensioned to engage the cladding 29. Flanges 97 extend from the seats95. The jaws 93 are abutted and secured by bolts through the flanges 97with the seats 95 engaging the upper end of the cladding 29 in goodelectrical contact. The strips 89 are bolted to the jaws 93 at theirlower end and to the plate 87 at their upper end. Current is supplied tothe cladding 29 through the feed-through 67, the clevis 77, conductors85, plate 87, strips 89 and clamping unit 91.

The lower-terminal assembly 65 is similar to the upper-terminal assembly63 except that the lower assembly 65 does not include the expansionjoint. Instead, the eyelet 75 is secured to an eyelet 99 extending froma cap 101 mounted on the tapered end 103 (FIG. 1) of the end plug 17 ofthe coolant-displacement rod 11.

In the practice of this invention, the coolant-displacement rod 11 withits cladding 29 and the tube 39 is mounted within the vessel 51 enclosedin a thermally insulating cylinder 105 (FIG. 4). The cylinder 105 hasinternal ceramic buttons 107 for centering the rod 11. Initially, thecylinder 51 is exhausted through exhaust outlet 61 by a "roughing" pumpoperated typically for about one-half hour. The pressure in the vessel51 is reduced typically to about 500 to 1000 microns.

Next, current, typically about 200 amperes, is supplied through theterminal assemblies 63 and 65. Since the zirconium oxide pellets 13 areelectrical insulators, the current is conducted through the cladding 29.The cladding is heated to about 1000° F. At this temperature the outersurface of the cladding is oxidized by the residual oxygen in the vessel51 which is just sufficient to accomplish this purpose. With thecladding maintained at 1000° F., an inert gas, typically argon, isadmitted through inlet 59 to increase the pressure in the vessel 51typically to about 1500 to 1600 pounds per square inch. With thecladding at the elevated temperature, its yield strength is reduced andthe pressure in the vessel is sufficient to collapse the cladding 29 onthe pellets 13. Since the pressure is isostatic, i.e., uniformthroughout, the cladding is collapsed uniformly. The gas is thenexhausted through outlet 61, the vessel 51 is disassembled and theprocessed coolant-displacement rod is removed.

While preferred embodiments of this invention are disclosed herein, manymodifications thereof are feasible. This invention is not to berestricted except insofar as is necessitated by the spirit of the priorart.

I claim:
 1. The method of processing a coolant-displacement rod of anuclear reactor, preparatory to the installation of said rod in saidnuclear reactor, to preclude the formation undesirably of an unsupportedpermanent axial gap in the cladding of said rod over the anticipatedtime of use of the rod in said nuclear reactor by reason of neutronbombardment of the rod, which gap would be collapsible under the forceexerted by the coolant of said nuclear reactor, said rod including astack of zirconium-oxide pellets in cladding of an alloy of zirconium,the inner surface of said cladding prior to said processing being spacedfrom the outer surfaces of said pellets; the said method including:mounting said rod in a container and evacuating said container fromatmospheric pressure to a pressure such that said rod is in anattenuated atmosphere containing residual oxygen not combined chemicallywith any other chemical element, thereafter conducting electricalcurrent only through said cladding to heat said cladding to atemperature at which the yield strength of said cladding issubstantially reduced and, while said rod is maintained at saidtemperature adding a gas to apply pressure uniformly to said cladding ofa magnitude sufficient at said temperature to collapse said claddinguniformly on the outer surface of said pellets of said stack while saidrod maintains its radial symmetry, the outer surface of the claddingbeing oxidized at said temperature by said residual oxygen, the saidprocessing, precluding the formation, by exposure to neutron flux, overthe period of time over which said rod will later be in use in areactor, of an unsupported axial gap in said cladding which wouldcollapse under the pressure and at the temperature, of the coolant ofsaid nuclear reactor.
 2. The method of claim 1 including applyingisostatic pressure to collapse the cladding.
 3. The method of processingan element of a nuclear reactor, preparatory to installation of saidelement in said nuclear reactor, to preclude the formation undesirablyof an unsupported permanent axial gap in the cladding of said elementover the anticipated time of use of the element in said nuclear reactorby reason of neutron bombardment of the element, which gap would becollapsible under the force exerted by the coolant of said nuclearreactor, said element including a stack of separate pellets enclosed inmetallic cladding, the inner surface of the cladding prior to saidprocessing being spaced from the outer surfaces of the pellets; the saidmethod including: mounting said element in a container and evacuatingsaid container from atmospheric pressure to a pressure such that saidelement is in an attenuated atmosphere containing residual oxygen notcombined chemically with any other chemical element, thereafterconducting electrical current only through said cladding to heat saidcladding to a temperature at which the yield strength of said claddingsubstantially reduced and while said element is maintained at saidtemperature, adding a gas to apply pressure uniformly to said claddingof a magnitude sufficient at said temperature to collapse said claddinguniformly on the outer surface of said pellets of said stack while saidelement maintains its radial symmetry, the outer surface of saidcladding being oxidized at said temperature by said residual oxygen, thesaid processing, precluding the formation, by exposure to neutron flux,over the anticipated period of time over which said element will laterbe in use in said nuclear reactor, of an unsupported axial gap on saidcladding which would collapse under the pressure or at the temperatureof the coolant of said nuclear reactor.
 4. The method of claim 3including applying isostatic pressure to collapse the cladding.
 5. Themethod of processing an element of a nuclear reactor, preparatory toinstallation of said element in said nuclear reactor, to preclude theformation undesirably of an unsupported permanent axial gap in thecladding of said element over the anticipated time of use of the elementin said nuclear reactor by reason of neutron bombardment of the element,which gap would be collapsible under the force exerted by the coolant ofsaid nuclear reactor, said element including a stack of pellets enclosedin said cladding, the inner surface of the cladding prior to saidprocessing being spaced from the outer surfaces of the pellets, saidpellets being composed of one or more of the class consisting ofuranium, plutonium, thorium and their compounds, zirconium oxide, andboron and its compounds and said pellets being of the samecross-sectional area so that their outer surfaces are coextensiveforming a surface of the same radius throughout the length of the stack,said cladding consisting of one of the class of an alloy of zirconiumand of stainless steel; the said method including: mounting said elementin a container and evacuating said container from atmospheric pressureto a pressure such that said element is in an attenuated atmospherecontaining residual oxygen not combined chemically with any otherchemical element, thereafter conducting electrical current only throughsaid cladding to heat said cladding to a temperature at which the yieldstrength of said cladding is substantially reduced and, while saidelement is maintained at said temperature adding a gas to apply uniformpressure to said cladding of a magnitude sufficient at said temperatureto collapse said cladding uniformly on the outer surfaces of saidpellets while each said pellet maintains its radial symmetry, the outersurface of said cladding being oxidized at said temperature by saidresidual oxygen, the said processing, precluding the formation, byexposure to neutron flux, over the anticipated period of time over whichsaid element will later be in use in a nuclear reactor, of anunsupported axial gap in said cladding which would collapse under thepressure, and at the temperature, of said coolant of said nuclearreactor.
 6. The method of claim 5 of processing a cylindrical elementwhich is radially symmetric including applying isostatic pressure tocollapse the cladding.
 7. The method of processing a control element ofa nuclear reactor, preparatory to installation of said element in saidnuclear reactor, to preclude the formation undesirably of an unsupportedpermanent axial gap in the cladding of said element over the anticipatedtime of use of the element in said nuclear reactor by reason of neutronbombardment of the element which gap would be collapsible under theforce exerted by the coolant of said nuclear reactor, said elementincluding a stack of pellets enclosed in cladding, the inner surface ofthe cladding prior to said processing being spaced from the outersurfaces of the pellets, said pellets being composed of one or more ofthe class consisting of zirconium oxide and boron and its compounds andsaid pellets being of the same cross-sectional area so that their outersurfaces are coextensive forming a surface of the same radius throughoutthe length of the stack, said cladding consisting of one of the class ofan alloy of zirconium and of stainless steel; the said method including:mounting said element in a container and evacuating said container fromatmospheric pressure to a pressure such that said element is in anattenuated atmosphere containing residual oxygen not combined chemicallywith any other chemical element, thereafter conducting electricalcurrent only through said cladding to heat said cladding to atemperature at which the yield strength of said cladding issubstantially reduced and, while said element is maintained at saidtemperature adding a gas to apply uniform pressure to said cladding of amagnitude sufficient at said temperature to collapse said claddinguniformly on the outer surfaces of said pellets while said elementsmaintains its radial symmetry, the outer surface of said cladding beingoxidized at said temperature by said residual oxygen; the saidprocessing, precluding the formation, by exposure to neutron flux, overthe anticipated period of time over which said element will later be inuse in a nuclear reactor, of an unsupported axial gap in said claddingwhich would collapse under the pressure, and at the temperature, of saidcoolant of said nuclear reactor.